The present invention relates to an in-mold foaming article of a polypropylene resin composition which is foamed in mold, and used for insulation materials, cushioning packing materials, delivery boxes; automobile parts such as a bumper core material, pillar, platform, and side projection material; pallet materials, tool boxes and the like. More particularly, the present invention relates to an in-mold foaming article of a polypropylene resin composition which is foamed in mold having high density and excellent in both mechanical strength properties of compressive strength and tensile strength.
A foamed article of a polypropylene resin is excellent in chemical resistance, heat resistance, strain recovering property after compression and the like compared to polystyrene resin foamed article, and widely used for cushioning packing materials, delivery boxes; automobile parts such as a bumper core material, pillar, platform, and side projection material; pallet materials, tool boxes and the like.
Conventionally, for production of a polypropylene resin pre-foamed particle giving the a polypropylene resin foamed article, there have been used volatile foaming agents typically including hydrocarbon foaming agents such as CFC-12, HCFC-22, HCFC-142b, and HFC-134a; hydrocarbon foaming agents such as propane, butane, and pentane. However, since any of these agents cause global environmental problems such as decomposition of the ozone layer and globe temperature rising, and safety problems inevitably occurring due to inflammability thereof, there have been recently used inorganic gas foaming agents such as a nitrogen gas, air, carbon dioxide gas for necessity of countermeasures against such problems (for example, Japanese Unexamined Patent Publication Nos. 6-192462/1994, 7-173321/1995, 8-259724/1996 and the like). Among them, particularly when a carbon dioxide gas is used as a foaming agent, this gas has excellent compatibility with a polypropylene resin and production of a pre-foamed particle having desired physical properties and of an internal foamed article comprising the particle is relatively easy. Therefore, there are widely known arts regarding a polypropylene resin pre-foamed particle obtained by using a carbon dioxide gas as a foaming agent and a foamed article comprising the particle.
By the way, it is also well known that a carbon dioxide gas is a cause of global temperature rising, and in arts using an inorganic gas foaming agent, the above-mentioned global environmental problems are not yet solved to date.
In view of such current conditions, investigations regarding selection of foaming agents for also solving global environmental problems have been conducted. And development of a polypropylene resin pre-foamed particle obtained by using, as a forming agent, water which had not been admitted as a foaming agent at all, and development of a foamed article comprising the particle have become successful and a lot of applications have been filed until now (for example, Japanese Unexamined Patent Publication Nos. 10-245437/ 1998, 11-21368/ 1999, 11-100458/1999 and the like).
Any of these arts is an epoch-making art giving a polypropylene resin pre-foamed particle unharmful to global environment and an internal foamed article comprising this particle, which can completely solve the above-mentioned problems.
Among other, an art described in Japanese Unexamined Patent Publication No. 11-100458/1999, for example, has an excellent feature which reverses conventional common knowledge in foamed articles using a polypropylene resin as a base material resin. Under this feature, tensile strength can be kept high even if fusion ratio of a foamed article is at most 70% which can not conventionally be imagined. And resultantly, improvement in productivity in internal molding and stability of quality can be attained simultaneously while maintaining the quality of the foamed article at the same level or more.